The improvement of voluntary muscle strength can be accomplished by an enlargement of muscle mass or by changes that occur in the nervous system. The neural mechanism underlying the increase in strength are poorly understood. A recent study has demonstrated that the voluntary strength of a hand muscle can be increased by training with imagined maximal contractions. This finding indicates that the nervous system is able to increase strength in the absence of muscle exercise and the origin of the neural change associated with strength training appears to be in the brain. The current application proposes to explore this phenomenon by studying two specific aims. Aim 1 will determine the effect of imagined-muscle-contraction training on the strength of the elbow flexor and abductor digiti minimi muscles. The basis for Aim 1 is that the control scheme for a hand muscle differs in several ways from a proximal muscle. Aim 2 will identify the underlying neural mechanisms for the improved muscle performance due to the imagined-muscle-contraction training. Preliminary results demonstrated that: (1) the strength of the elbow flexor muscles can be increased (40%, an average of 3 subjects) by the imagined-contraction training; (2) there was a strong activation in the prefrontal association cortex during the imagined contraction and this activation was stronger in trained subjects; and (3) the activation pattern of the brain during an imagined contraction was similar to that during a forceful muscle contraction. The hypotheses of this application are that the imagined-contraction training will cause a similar increase in strength in the fifth finger abductor and elbow flexor muscles and that this increase in muscle force will be caused by a greater descending command from the prefrontal cortex. Three groups (one control and two training groups) of young, healthy human subjects will be recruited for the study. The training groups will be trained for 40 sessions with the imagined-maximal contractions. Voluntary strength of the two muscle groups will be evaluated before, during, and after training. Functional magnetic resonance imaging will be used to identify the activated brain regions during the imagined and actual maximal contractions and to quantify the magnitude of the brain and muscle activation before and after training. The project will provide information on the mechanisms of human voluntary muscle strengthening and on the understanding of how willed actions occur. The training method may lead to a rehabilitation regimen for maintaining and/or restoring motor functions in patients who cannot perform forceful muscle contractions.